`exactextractr`

is an R package that quickly and accurately summarizes raster values over polygonal areas, commonly referred to as *zonal statistics*. Unlike most zonal statistics implementations, it handles grid cells that are partially covered by a polygon. Typical performance for real-world applications is orders of magnitude faster than the `raster`

package.

.

Calculations are performed using the C++ `exactextract`

tool. Additional background and a description of the method is available here. Full package reference documentation is available here.

The package provides an `exact_extract`

method that operates analogously to the `extract`

method in the `raster`

package. The snippet below demonstrates the use of this function to compute a mean December precipitation for each municipality in Brazil.

```
library(raster)
library(sf)
library(exactextractr)
# Pull municipal boundaries for Brazil
brazil <- st_as_sf(getData('GADM', country='BRA', level=2))
# Pull gridded precipitation data
prec <- getData('worldclim', var='prec', res=10)[[12]]
# Find the mean precipitation amount for each municipality
brazil$mean_prec <- exact_extract(prec, brazil, 'mean')
# Find min and max precipitation amount in a single pass
brazil[, c('min_prec', 'max_prec')] <- exact_extract(prec, brazil, c('min', 'max'))
```

`exactextractr`

can summarize raster values using several pre-defined operations as well as arbitrary R functions. Pre-defined operations are specified by providing one or more operation names to the `fun`

parameter of `exact_extract`

.

The following summary operations are supported:

Name | Description |
---|---|

`count` |
Sum of all cell coverage fractions. |

`majority` (or `mode` ) |
The raster value with the largest sum of coverage fractions. |

`max` |
Maximum defined value of cells that intersect the polygon, ignoring coverage fractions. |

`mean` |
Mean defined value of cells that intersect the polygon, weighted by the percent of the cell that is covered. |

`min` |
Minimum defined value of cells that intersect the polygon, ignoring coverage fractions. |

`minority` |
The raster value with the smallest sum of coverage fractions. |

`sum` |
Sum of defined values of raster cells that intersect the polygon, with each raster value weighted by its coverage fraction. |

`variety` |
The number of distinct raster values in cells wholly or partially covered by the polygon. |

Two additional summary operations require the use of a second weighting raster, provided in the `weights`

argument to `exact_extract`

Name | Description |
---|---|

`weighted_mean` |
Mean defined value of cells that intersect the polygon, weighted by the product of the coverage fraction and the value of a second weighting raster. |

`weighted_sum` |
Sum of defined values of raster cells that intersect the polygon, multiplied by the coverage fraction and the value of a second weighting raster. |

These operations are described in more detail below.

`exact_extract`

allows for calculation of summary statistics based on multiple raster layers, such as a population-weighted temperature. For the `weighted_mean`

and `weighted_sum`

operations, this is as simple as providing a weighting `RasterLayer`

to the `weights`

argument of `exact_extract`

. The weighting raster must use the same coordinate system as the primary raster, and it must use a grid that is compatible with the primary raster. (The resolutions and extents of the rasters need not be the same, but the higher resolution must must be an integer multiple of the lower resolution, and the cell boundaries of both rasters must coincide with cell boundaries in the higher-resolution grid.)

One application of this feature is the calculation of zonal statistics on raster data in geographic coordinates. The previous calculation of mean precipitation amount across Brazilian municipalities assumed that each raster cell covered the same area, which is not correct for rasters in geographic coordinates (latitude/longitude).

We can correct for varying cell areas by creating a second raster with the area of each cell in the primary raster, and using this raster in a `weighted_mean`

summary operation. (The `area`

function from the `raster`

package will calculate the cell areas for us.)

`brazil$mean_prec_weighted <- exact_extract(prec, brazil, 'weighted_mean', weights=area(prec))`

With the relatively small polygons used in this example, the error introduced by assuming constant cell area is negligible. However, for large polygons that span a wide range of latitudes, this may not be the case.

In addition to the summary operations described above, `exact_extract`

can accept an R function to summarize the cells covered by the polygon. Because `exact_extract`

takes into account the fraction of the cell that is covered by the polygon, the summary function must take two arguments: the value of the raster in each cell touched by the polygon, and the fraction of that cell area that is covered by the polygon. (This differs from `raster::extract`

, where the summary function takes the vector of raster values as a single argument and effectively assumes that the coverage fraction is `1.0`

.)

An example of a built-in function with the appropriate signature is `weighted.mean`

. Some examples of custom summary functions are:

```
# Number of cells covered by the polygon (raster values are ignored)
exact_extract(rast, poly, function(values, coverage_fraction)
sum(coverage_fraction))
# Sum of defined raster values within the polygon, accounting for coverage fraction
exact_extract(rast, poly, function(values, coverage_fraction)
sum(values * coverage_fraction, na.rm=TRUE))
# Number of distinct raster values within the polygon (coverage fractions are ignored)
exact_extract(rast, poly, function(values, coverage_fraction)
length(unique(values)))
# Number of distinct raster values in cells more than 10% covered by the polygon
exact_extract(rast, poly, function(values, coverage_fraction)
length(unique(values[coverage_fraction > 0.1])))
```

A multi-raster summary function can also be written to implement complex weighting behavior not captured with the `weighted_mean`

or `weighted_sum`

summary operations. If `exact_extract`

is called with a `RasterStack`

instead of a `RasterLayer`

, the R summary function will be called with a data frame of raster values and a vector of coverage fractions as arguments. Each column in the data frame represents values from one layer in the stack, and the columns are named using the names of the layers in the stack.

A more verbose equivalent to the `weighted_mean`

usage demonstrated could be written as:

```
stk <- stack(list(prec=prec, area=area(prec)))
brazil$mean_prec_weighted <-
exact_extract(stk, brazil, function(values, coverage_frac)
weighted.mean(values$prec, values$area*coverage_frac, na.rm=TRUE))
```

Note that the assembly of a `RasterStack`

to use an R summary function requires that the primary and weighting rasters share an extent and resolution, a limitation not shared by the named summary operations.

`exactextractr`

can also rasterize polygons though computation of the coverage fraction in each cell. The `coverage_fraction`

function returns a `RasterLayer`

with values from 0 to 1 indicating the fraction of each cell that is covered by the polygon. Because this function generates a `RasterLayer`

for each feature in the input dataset, it can quickly consume a large amount of memory. Depending on the analysis being performed, it may be advisable to manually loop over the features in the input dataset and combine the generated rasters during each iteration.

An example benchmark using the example data is shown below. The mean execution time for `exactextractr`

was 2.6 seconds, vs 136 for `raster`

. Timing was obtained from execution on an AWS `t2.medium`

instance.

```
microbenchmark(
a <- exact_extract(prec, brazil, weighted.mean),
b <- extract(prec, brazil, mean, na.rm=TRUE), times=5)
# Unit: seconds
# expr min lq mean median uq max neval
# a <- exact_extract(...) 2.5674 2.586868 2.626761 2.587283 2.613296 2.778957 5
# b <- extract(...) 136.1710 136.180563 136.741275 136.226435 136.773627 138.354764 5
Although `exactextractr` is fast, it is still several times slower than the
command-line [`exactextract`](https://github.com/isciences/exactextract) tool.
Results from `exactextractr` are more accurate than other methods because raster
pixels that are partially covered by polygons are considered. The significance
of partial coverage increases for polygons that are small or irregularly shaped.
For the 5500 Brazilian municipalities used in the example, the error introduced
by incorrectly handling partial coverage is less than 1% for 88% of
municipalities and reaches a maximum of 9%.
```

Installation requires version 3.5 or greater of the GEOS geometry processing library. It is recommended to use the most recent released version (3.8) for best performance. On Windows, GEOS will be downloaded automatically as part of package install. On MacOS, it can be installed using Homebrew (`brew install geos`

). On Linux, it can be installed from system package repositories (`apt-get install libgeos-dev`

on Debian/Ubuntu, or `yum install libgeos-devel`

on CentOS/RedHat.)